The distribution and kinematics of neutral hydrogen in spiral galaxies of various morphological types

The two basic classes of galaxies are the ellipticals and the disk galaxies. Disk galaxies generally possess a nuclear bulge, which is similar to ellipticals in many respects, and a disk in which usually spiral structure can be seen. Disk galaxies without spiral structure are classified as lenticulars (S0's), the remaining ones are called spirals. Unlike the ellipticals, which form a homogeneous class of galaxies, the spirals can be subdivided according to several, often complementary, criteria. This has led to the distinction of Hubble types (Sa, Sb, Sc, Sd, Sm), ordinary and barred spirals (SA and SB), and varieties (ring and spiral variety). Many spirals have been classified according to this scheme, or slightly more elaborate versions of it. Other classification criteria have been: the morphology of the arms, which is related to the luminosity, and the degree of concentration of light towards the centre, which is related to the integrated spectral type. Irregular galaxies of the magellanic type (Im) form the natural endpoint of the so-called Hubble sequence E, So, Sa, Sb, Sc, Sd, Sm, Im. These classifications are widely used nowadays, but their physical significance is still poorly understood. In this thesis we try to make a contribution to a better understanding of the physical processes underlying the morphology of a spiral galaxy.

We have obtained new information, especially on the kinematical properties, for a number of spirals of various morphological types which enabled us to approach this problem in a new context. In this thesis we will discuss a sample of galaxies of various morphological types for which 21 cm line data are available at a spatial resolution such that the ratio radius/beamsize is larger than about 5. For a number of galaxies we have used information from the literature, for other galaxies we have used information from Westerbork Synthesis Radio Telescope (WSRT) studies performed by other investigators. We have observed a number of galaxies with the WSRT ourselves, most of them selected such as to achieve an even distribution of well-studied galaxies along the spiral sequence. The results of these 21-cm line observations have been compared with optical information available in the literature, and with recent results from surface photometry of a number of galaxies made by Van der Kruit and Bosma. Since these data have usually been obtained for a different purpose it should be clear that our sample is still limited in several respects.

The structure of this thesis is as follows: In chapter 2 we present the sample of spirals used in our study and we discuss the criteria which lead to the choice of objects we have observed with the WSRT. In chapter 3 we describe various problems associated with the processing of 21-cm HI line data obtained with the WSRT and subsequent interpretation of those data. In chapter 4 we have collected a series of articles on individual galaxies, some of them already published and others ready for submission to Astronomy and Astrophysics. In chapter 5 we present a description of the maps of the HI distribution and the radial velocity fields of the galaxies in the sample. Models of the mass distribution have been determined for 25 galaxies; these are presented in chapter 6. In chapter 7 we compare the results from these models with other physical quantities and try to relate them with morphological type. In chapter 8 we give a brief summary of our results and suggestions for future work. The main result of our work is that the simple working model for a spiral galaxy which we described above, needs a first order revision. Most spirals do not follow the usual picture of axisymmetric disks in differential rotation with small amplitude perturbations due to spiral arms. In a number of cases large scale symmetric deviations from axial symmetry are found, some of these seem to be associated with oval shaped (oval in the sense of broadly elliptical) structures in the plane of the disk, and others with a warping of the HI layers in the outer parts. Moreover, asymmetries occur in all galaxies, and in some cases these asymmetries have a very large amplitude. The mass models indicate that in the outer parts of a spiral the mass-to-light ratio is higher than in the inner parts. Perhaps a substantial fraction of the mass is not distributed in a disk at all. The ratio of total mass to neutral hydrogen mass tends to remain more or less constant in the outer parts. There is no simple relation between the morphological type of a galaxy and the mass distribution.